DESCRIPTION (FROM APPLICANT'S ABSTRACT): The overall goal of this proposal is to identify the function of HIV-1 Vif and determine the role of Vif in the virus life cycle. The mechanism of Vif function is unknown. Vif-defective viruses enter cells normally but are defective in their ability to synthesize viral DNA. Vif acts to enhance HIV-1 infectivity in virus producer cells, most likely by affecting events during virus assembly. Vif-defective virions exhibit a morphological abnormality in the virion core and are defective in their ability to synthesize viral DNA in the endogenous reverse transcriptase reaction. Our preliminary studies suggest that this effect of Vif on viral DNA synthesis results indirectly from an effect of Vif on the stability of the virion core. Recent reports and our preliminary studies suggest that Vif acts to overcome an endogenous inhibitor of virus infectivity that is present in non-permissive cells. Aim 1 will identify and characterize a 40 KD cellular protein we find to be tightly associated with Vif in non-permissive but not permissive cells and investigate its functional role. We will investigate whether additional cellular factors are associated with Vif in a complex we find in the cytoplasm of non-permissive cells. We will also investigate the role of host cell factors in the species restriction to HIV-1 Vif function. Aim 2 will investigate the nature of the defect in Vif-negative HIV-1 virions. These studies will use in vitro biochemical and structural approaches to examine the role of Vif in the stability and disassembly of the virion core. We will also analyze the effect of Vif on the composition and stability of uncoated viral nucleoprotein complexes isolated from the cytoplasm of acutely infected cells in vivo. Aim 3 will biochemically characterize the native HIV-1 Vif protein and its association with a high molecular weight complex in the cytoplasm of infected cells. Vif mutants will be used to identify the domains of Vif responsible for its interaction with the complex and to determine the functional significance of the complex ford virus replication. We will also investigate the functional significance of Vif phosphorylation and regulation by cellular kinases. These studies will elucidate mechanisms of Vif function and also have broader relevance for understanding other critical aspects of the virus life cycle that are potential therapeutic targets.